What are credible speed limits?

Answer

Credible speed limits are limits that meet the expectations raised by the road and the road environment, so that drivers are more inclined to comply with them [25]. In other words, credible speed limits are felt to be logical. Thus, it would mostly be rather incredible to lower the limit of an 80km/h or a 50km/h road to 60 and 30 km/h respectively by merely changing the speed limit signs without adapting the road. In most cases, the lower limits should be supported by adapting the road or the road environment. Table 3 shows a few characteristics affecting the credibility of a speed limit, since they automatically invite driving at a higher or lower speed. Explaining why a limit does not match the appearance of the road may help improve credibility (for instance ‘Noise pollution’ or ‘School area’)

Obviously, road safety will always be the guiding principle: first determine what safe speed limit a road requires (see the question What are safe speeds?), and subsequently ensure that the limit is credible.

Road characteristic

Effect of driving speed

Number of lanes

More lanes → higher speed

Road/lane width 

Wider roads/lanes → higher speed

Median 

Median present → higher speed 

Hard shoulder

Hard shoulder present → higher speed

Longitudinal marking 

Edge and centre line marking → higher speed

Road surface 

Smooth road surface (asphalt) → higher speed

Open environment

Open environment → higher speed

Table 3. Road and road environment characteristics that imperceptibly affect driving speed (Source: [26]).